U.S. Pat. No. 5,486,830 (incorporated by reference herein) describes a system wherein a synthetic aperture radar (SAR) apparatus carried on an airborne platform transmits a set of pulses for reception at a coherent gain-block tag. The tag processes each received pulse, first by amplitude-modulating the pulse (denoted as “chopping”), and then by applying bi-phase modulation. The tag then transmits the result (i.e., the processed pulse) to the SAR apparatus. The tag thus functions as a transponder apparatus. The processed pulse that is transmitted by the tag is also referred to as a retransmitted pulse or tag response.
The SAR apparatus collects a series of the retransmitted pulses and performs coherent SAR-tag processing. A tag image is produced that can be used to provide relative location of the tag within a normal SAR image. The SAR apparatus also forms its normal SAR context image using reflections of the same set of radar pulses received by the tag. In addition, the radar can process the retransmitted pulses to extract data from the tag.
In the system of U.S. Pat. No. 5,486,830, the amplitude modulation applied by the tag causes a significant distortion in the retransmitted pulses. This distortion manifests itself as unwanted side lobes that appear when the SAR apparatus applies range-compression to the retransmitted pulses. The first harmonic of these side lobes contains half the energy of the desired, range-compressed signal. When several tags are being illuminated by the SAR apparatus, it becomes difficult to distinguish the unwanted side lobes associated with any given tag from the main lobes (primary response lobes) of other tags. This effect makes it difficult for the SAR apparatus simultaneously to process several tags within a common set of radar pulses.
It is desirable in view of the foregoing to provide for reduction of distortion in the retransmitted pulses produced by transponders in systems of the type described above. Exemplary embodiments of the present invention reduce distortion in retransmitted pulses by applying a random delay to a chopping signal used for modulation in the transponder. This permits the SAR apparatus to process simultaneously many transponders within a common area of illumination.